We are studying factors affecting transmission through a spinal reflex circuit in the mammalian central nervous system. The reflex chosen for investigation is the plantar cushion (PC) reflex, a cutaneous reflex in the cat. The basic neural circuit of the PC reflex appears to consist of not more than two serially connected dorsal horn cells, interposed between the cutaneous afferents from the plantar cushion and motoneurons innervating the intrinsic plantar muscles. We intend to continue our analysis of the circuit of this reflex, using the intracellular horseradish peroxidase technique. Intracellular recordings from first-order dorsal horn cells responding to PC stimulation will be made during iterated stimulation, to investigate whether the changes in reflex transmission during iterated stimulation that we have previously described are due primarily to pre- or to postsynaptic mechanisms. We shall continue our studies of classical conditioning of the PC reflex, concentrating on unanesthetized, decerebratespinal preparations. If specific conditioning of the PC reflex can be demonstrated convincingly, we shall use the PC reflex to study neural mechanisms related to changes in transmission during conditioning.